Introduction to Python Programming

Transcription

1 Introduction to Python Programming (3) Variables, Types, Operators S. Thater and A. Friedrich Saarland University Winter Semester 2011/2012 S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

2 Revision What is an algorithm? What is a program? Requirements for algorithms? What is compilation? What is interpretation? What does platform independence mean? S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

3 Imperative Programming Paradigm Imperative: First do this, then do this. Procedural Programming. Control Structures execute computational steps, state of the program changes as a function of time. Commands can be grouped into procedures. Example Celsius_to_Fahrenheit(c) 1 Multiply c with 1.8 and save result to temp. 2 Add 32 to temp and return result of this. S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

4 Elements of imperative programs Variables Assignments Expressions Control Structures: loops, branches S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

5 Values, Variables, Data Types Values may have different data types: numbers, lists, strings... Variable Assignment list = [1, 2, 3, 4] number = 4 text = hello number = world Variables = placeholders for values. Variables point to positions in the memory where values are stored. Value of a variable can change over time. S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

6 Some Data Types Boolean: truth values: True and False Numbers: int (2), float (2.0), complex Strings: str Collections: tuple, list, set, dict S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

7 Dynamic Typing Variables in Python do not have fixed data types. The type of a variable is the assigned value s data type. During runtime, a variable can take values of different types. Dynamic Typing >>> x = 15.4 >>> type(x) <type float > >>> x = "Python is great!" >>> type(x) <type str > S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

8 Floating Point Numbers Decimal numbers are represented as floats (1.1, 47.11) Range depends on system CAREFUL! Often, the internal representation of floating point numbers is imprecise. >>> What to do about this? use ε when comparing floating point numbers. >>> epsilon = >>> x_equal_y = abs(x-y) < epsilon >>> x_equal_y True S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

9 Expressions Expressions = constructs describing a value We distinguish: Literals = expressions from / in which the value can be directly read / written, e.g. 1.0, True, Hello World Variables = references to values Complex expressions with operators, e.g. 3+5 Calls of functions or methods, e.g. find_max(l1) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

10 Elementary Arithmetic Operators Addition Subtraction Multiplication Division Modulo a + b a - b a * b a / b a % b If a and b do not have the same type, the operations result in a value of the more general type. Example >>> a = 1 >>> b = 2.4 >>> a + b What are the types in this example? Which type is more general? Why? S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

11 Precedence Expressions may contain multiple operators: 3 + 2*4 Precedence = order in which operators are evaluated Standard precedence rules: multiplication/division before addition/subtraction Parentheses indicate precedence directly Example 3 + (2*4) = 11 (3+2) * 4 = 20 Style: sometimes it is recommended to use parentheses even if they are redundant (legibility) Don t use parentheses when precedence is irrelevant, e.g is better than 2+(3+4) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

12 Truth Values (Boolean) The type bool represents the two truth values True and False negation conjunction disjunction not a a and b a or b On board: truth tables Precedence: not > and > or a and not b or c = (a and (not b)) or c Short-circuit evaluation: the evaluation stops as soon as the result is evident (True or...) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

13 String Literals Strings are sequences of characters (no separate type for characters). Example "This is a string." That, too. "He said \"Hello.\"." He said "Hello". String may not contain any special characters (umlauts etc.) if no encoding is specified. (in Python 2.x - no problems in Python 3.x!) Encoding is specified in the first code line: # -*- Coding: utf-8 -*- # -*- Coding: latin-1 -*- S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

14 Lists We can have lists of numbers: numberlist = [1, 2, 3, 4] Or lists of strings: weekdays = [ Mon, Tue, Wed, Thu, Fri, Sat, Sun ] We can access individual items of the list using brackets: 1 >>> print(weekdays[2]) 2 Wed 3 >>> print(weekdays[6]) 4 Sun Index List Mon Tue Wed Thu Fri Sat Sun S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

15 Exercise 1: Types Name the types of the following values. (a) 1.0 (b) a (c) False (d) 5 (e) [ hello, world ] (f) c (g) Python: (h) [2.5, 6.7, 1.2, 4] S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

16 Some String Operators Concatenation: Hello + World HelloWorld Access to individual characters with list indices: Hello [0] ==> H Hello [1] ==> e Test whether a substring occurs: He in Hello ==> True Ha in Hello ==> False Length: len( Hello ) = 5 S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

17 Relational Operators less than greater than less than or equal to greater than or equal to Equal to not equal to a < b a > b a <= b a >= b a == b a!= b The result of such a comparison is a boolean. Example >>> 3 > 2 True >>> (2*3) + 4!= 2*3 + 4 False S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

18 Variables Placeholders for values one can assign the value of an expression to variables variables can be evaluated in order to use their value in an expression print() is a function that prints the value of an expression to the console (the standard output) Example >>> number = 123 >>> number = number + 2 >>> print(number) 125 S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

19 Variables Variables (more generally, all identifiers) must start with a letter or _. The remainder may include digits. umlauts etc. are not allowed (ASCII encoding) (actually allowed in Python but better stick to ASCII anyways) the name must not be a keyword (if, while etc) the names are case-sensitive convention: variables start with a lower-case letter Which ones are allowed/recommended? foo, 2foo, foo12, _foo, i f, überzwerg S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

20 Assignments I var = expr the expression expr is evaluated, then its value is stored in var. var 1 = var 2 =...= expr the value of expr is assigned to all variables var i (all variables point to the same value in the memory) Example >>> a = b = 6.0/4.0 >>> print(a) 1.5 >>> print(b) 1.5 S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

21 Assignments II var 1, var 2,..., var n = expr 1, expr 2,..., expr n all expr i are evaluated, then the corresponding values are assigned to var i Example >>> a, b = 6.0/4.0, Hello + World >>> print(a) 1.5 >>> print(b) HelloWorld S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

22 Assignments Long version x = x + expr x = x - expr x = x * expr x = x / expr x = x % expr Shorthand x += expr x -= expr x *= expr x /= expr x %= expr S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

23 Exercise 2: Assignments The following listing shows several steps of a program. For each step, write down the values and types of a, b and c. (a) a, b = 5, 3 (b) c = test (c) a = a % b (d) b *= b (e) a = c[1] (f) c = a = b = True (g) c = not(a or b) (h) a = b or c (i) b = str(a) + " Love" S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

24 Type Conversion in Python Reading a String from the Console variable = input(prompt) Anything we read from the console is a string, even if we enter 42.0 For computation, we need to convert this to some number. 1 variable = input("enter a number: ") 2 int_number = int(variable) 3 float_number = float(variable) Conversion of numbers (or other types) to strings works like this: text = str(number) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

25 Exercise 3: Car Stopping Distance Write a program car_stopping_dist.py that computes the stopping distance of a car using the following rule of thumb. (The only input to your program is the velocity, which is to be read from the console.) Breaking distance = (velocity / 10) * (velocity / 10) Reaction distance = (velocity / 10) * 3 Stopping distance = reaction distance + breaking distance Check whether your program is correct using the following numbers: Velocity Stopping Distance 80.0kmh/h 88.0m 50.0kmh/h 40.0m 100.0kmh/h 130.0m S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

26 Exercise 4: Volume of a Cone Write a program cone_volume.py that computes the surface area and the volume of a cone. Radius and height are to be read from the console. SurfaceArea = π r h + π r 2 Volume = 1 3 π r 2 h Radius Height Surface Area Volume HINT: You can use the following lines of code when using π: 1 import math 2 print(math.pi) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

27 Exercise 5: Celsius to Fahrenheit Converter Celsius to Fahrenheit Fahrenheit = Celsius* Write a program celsius_fahrenheit.py that reads a degree Celsius and outputs the corresponding value in degrees Fahrenheit. The Celsius value is to be read from the command line. S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

28 Exercise 6: Indian Takeaway 1 Achari Paneer 2 Gajar Ka Achar 3 Aloo Dum 4 Kabuli Chana 5 Baingan Bharta 6 Apple Jalebi In an Indian restaurant, the menu items are labeled with numbers, which the customers use to order their dishes. Your job is to translate these numbers to the names of the dish for the cooks. Write a program indian_takeaway.py that performs this task. Sample Output >>> INDIAN TAKEWAY! Please enter the number of your dish: 4 Thank you for ordering Kabuli Chana S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29

29 Exercise 7: What is the output of the following program? Explain what happens here! 1 x = (5 == 6) 2 y = str(x) 3 print(y[1]) S. Thater and A. Friedrich ( Saarland University) Introduction to Python Programming Winter Semester 2011/ / 29